A wire electric discharge machining apparatus applies a voltage to a gap formed between a wire electrode and a workpiece (a machined object) and causes electric discharge, thereby machining the workpiece. In the wire electric discharge machining apparatus, a machining fluid is interposed between the wire electrode and the workpiece to achieve insulation, cooling, and elimination of machined chips. The temperature of the machining fluid rises due to electric discharge caused between the electrode and the workpiece and heat generation of a pump that supplies the machining fluid to a work tank in which the workpiece is placed.
When the temperature of the machining fluid in the work tank rises, the workpiece, a table to which the workpiece is fixed, and a machine structure deform due to a thermal expansion, which leads to reduction in a machining accuracy or deterioration in a surface roughness. Furthermore, a cooling efficiency in an interpolar portion decreases and thus the wire electrode may be broken. Accordingly, the wire electric discharge machining apparatus has a cooling device that cools the machining fluid in the work tank.
To suppress reduction in the machining accuracy resulting from a thermal influence of the machining fluid, not only the rise in the temperature of the machining fluid is simply suppressed but also the machining fluid needs to be kept at a predetermined temperature. To execute a temperature control on the machining fluid, a temperature sensor is arranged in the work tank, for example. The machining fluid supplied into the work tank is cooled (temperature controlled) so that the temperature measured by the temperature sensor follows a target temperature.
For example, a conventional wire electric discharge machining apparatus includes a cooling device, a plurality of temperature sensors (first to third temperature sensors), and a fluid surface detector that detects the height of a fluid surface of the machining fluid. The first temperature sensor detects a temperature of the machining fluid cooled by the cooling device, the second temperature sensor detects a temperature of the work tank, and the third temperature sensor detects a machine temperature. This wire electric discharge machining apparatus selects one of the first and second temperature sensors based on a signal output by the fluid surface detector according to the height of the fluid surface. The wire electric discharge machining apparatus cools the machining fluid to cause the temperature of the machining fluid detected by the selected temperature sensor to approach a target temperature detected by the third temperature sensor (see, for example, Patent Literature 1).
Another conventional wire electric discharge machining apparatuses uses two types of machining fluids such as water and oil according to cases. This wire electric discharge machining apparatus adopts a method of cooling oil with water used as a refrigerant, for example. In this method, the oil machining fluid is likely to have a temperature higher than that of the water machining fluid. Accordingly, the wire electric discharge machining apparatus uses an offset feedforward amount to set a target temperature in the case of using oil lower than a target temperature in the case of using water. In a machining mode that uses oil, the offset feedforward amount is set to a higher value than that in a machining mode that uses water (see, for example, Patent Literature 2).